Asphalt mixes are widely used in road construction and maintenance and the majority of asphalt mixes that are used currently are produced by the hot method which is generally known as hot-mix or HMA and also known as asphalt concrete. These asphalt mixes consists of asphalt binder and mineral aggregate. The aggregates used could be either natural or processed. Normally processed aggregates are used which have been quarried, crushed, separated into distinct size fractions, washed or otherwise processed to achieve certain performance characteristics of the finished HMA. The aggregates are usually a mixture of various sizes to give desired load bearing strength and properties to the asphalt mix as specified in the mix design.
The strength and durability of the asphalt pavements depends on various factors such as the properties of the materials used, the interaction of the various materials and the mix-design. One of the key factors determining the strength and durability of the asphalt pavement also depends on the ability of the mix to be compacted to the desired design densities and air-voids. A mix that is not properly compacted will have poor strength and will be prone to various pavement distresses. It is important to attain proper coating of the aggregate with asphalt with optimum binder content and good adhesion of asphalt onto the aggregate and good cohesive strength of the asphalt to produce a mix that will have a good performance during the lifetime of the pavement. Pavement is designed to avoid some commonly known distresses such as permanent deformation, fatigue cracking, low temperature cracking and moisture damage.
The mixes are also designed to achieve a specified density and % air-voids. The temperature of the mix has a big influence on the ability to compact. Various grades of asphalt are used in asphalt mixes depending on the predicted traffic load and expected pavement temperatures. Higher PG (Performance Grade) asphalts are used in pavements with a higher traffic load and in areas where the pavement temperatures are higher. For example PG 76-22 asphalt is used on highways in the Southern part of US and in pavements with lower traffic loadings, PG 64-22 asphalts are used. With higher PG grades, binders are usually polymer modified (PMA) and are consequently more viscous requiring much higher mix temperatures to facilitate compaction to the target design densities. One of the important consequences of the higher asphalt temperatures is the substantial increase in asphalt fumes at the hot-mix plant and during construction which are major issues for the environment as well as personnel health. These days there is a major impetus by the asphalt industry to minimize the asphalt fumes to advance environmental stewardship.
Additional benefits in lowering mix and compaction temperatures depending on the technique used are lower fuel costs for the hot-mix producer, lower costs for emission control, lower emissions would permit paving in non-attainment areas where there is strict air pollution regulations. Warm mix asphalt will also allow longer haul distances as the mix at a lower temperature will maintain lower viscosity and workability when it reaches the construction site. Warm-mix asphalt paving could be done at cooler weather compared to hot-mix asphalt and thereby extending the asphalt season eg. paving late into Fall and paving earlier in Spring. Lower temperature also would reduce oxidative hardening of the asphalt which will enhance pavement performance in extending the pavement life.
There is a definite need for reducing the aggregate mixing, lay down and compaction temperatures while at the same time achieving the designed pavement air voids and density and reducing fume emissions to acceptable levels.
Moisture damage is also of great concern. Moisture damage in asphalt mixes can occur by two major pathways. First water will displace asphalt from the aggregate surface especially the ones containing higher amounts of silica since water has a higher affinity for the aggregate surface compared to asphalt and there is lack of chemical bonding of asphalt to the surface. This is known as stripping. Adhesion is the formation of chemical bond between asphalt and the aggregate. Secondly water over a period of time under repeated load can get inside asphalt and reduce the cohesive strength of asphalt. The results of stripping and loss of cohesive strength of the asphalt on the properties of the mix can be conveniently evaluated by the Hamburg wheel tracking test which measures deformation of the mix by a repeated load under water and by the Tensile strength Ratio test such as ASTM D 4867 procedure.
Several processes and products are being introduced into the market to reduce compaction and mix temperatures which are known as warm-mix technologies and the mixes are known as warm-mix asphalt. These techniques that have been introduced in the market to reduce the mixing and paving temperatures can be broadly classified into three categories. One such technology is the addition of products such a Fisher-Tropsch wax known as Sasobit promoted by Sasol GmbH International, which is a viscosity flow improver that reduces the viscosity of the aggregate mix, thereby reducing the mixing and compaction temperatures. Fischer-Tropsch wax being a plastomeric material suffers from the problems of asphalt binder embrittlement and consequent Low Temperature cracking fatigue as demonstrated by the Bending Beam Rheometer. This technique does not require a significant modification to the hot-mix plant.
A second category of treatments introduces certain amount of water into the mix by different means. When the temperature of the asphalt or the mix is higher than the boiling point of water, water evaporates and causing foaming of asphalt thereby increasing the surface area of asphalt significantly. The foaming process reduces the viscosity of the aggregate mix, thereby helping to produce the aggregate mix at reduced temperatures, which facilitates paving at lower than normal temperatures. The Eurovia Zeolite process works through the generation of foam by liberating water of hydration and in this way helps to generate the foam in asphalt. The MeadWestvaco Warm Mix process uses water from the emulsified asphalt to produce the same foaming effect. In the Shell WAM process water is directly introduced to aggregate hot mixing process to generate foaming of asphalt. These techniques require some modifications to the hot-mix plant. The concern with these moisture foaming technologies is the unknown long term effect of moisture damage since water is deliberately introduced into the mix.
The third category includes methods where there is a change in mechanical design of hot-mix plant that allows production of the mix at lower than normal temperatures and which can be paved at lower than normal temperatures.
On the negative side lower mix temperatures could result in less effective drying of the aggregate. The aggregates which normally contain varying amounts of water depending upon the aggregate stockpile storage location and moisture/rainfall that is prevalent in the area. The presence of water will impede proper bonding of asphalt to the aggregate surface and will result in moisture damage. This is also a concern that needs to be dealt with in the Warm Mix techniques that deliberately introduces water into the mix.
The present invention is concerned with the technical problem of reducing the mixing and paving temperatures and at the same time improving the resistance to moisture resistance of the hot-mix asphalt used for production of road surfaces without sacrificing the performance characteristics of the asphalt mix. More specifically, the present inventors have found that a novel combination of surfactants and rheology modifiers can improve the ease of mixing, lay down and compaction of asphalt mixes by reducing the viscosity of the asphalt binder and aggregate mix during the production and paving of the mix and thereby reduces the compactive effort required to attain the optimum design densities. The unique combination of surfactants that help in compaction also function as adhesion promoters by improving the coating and bonding of the asphalt to the aggregates surfaces. Asphalt binder modified with these surfactants has a higher affinity to aggregate surface compared to water and so water cannot displace or strip asphalt from the aggregate surface. The rheology modifiers also improve the cohesion strength of asphalt at pavement temperatures and thereby further improve the moisture resistance properties of the mix. This is the first instance that a unique combination of surfactants and rheology modifiers have been used as a single package that function as a compaction aid/warm additive and adhesion promoter in one. Unlike other additives and techniques for warm-mix, the current invention does not deliberately introduce water into the mix and does not have any adverse effect on low temperature properties of asphalt as demonstrated by the Bending Beam Rheometer.